1. Field of the Invention
This invention relates to a method for eliminating nonuniformity in sensitivity in an image read-out apparatus wherein a light beam is reflected and deflected by a rotating polygon mirror such that it scans a recording medium, on which an image has been recorded, in a main scanning direction, the recording medium is simultaneously moved with respect to the light beam in a sub-scanning direction approximately normal to the main scanning direction, the recording medium being thereby scanned with the light beam in two directions, and light is detected which is radiated out of the recording medium during the scanning and which represents the image.
2. Description of the Prior Art
Techniques for reading out an image, which has been recorded on a recording medium, in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields.
For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In the image recording and reproducing systems described above, an image read-out apparatus is used in order to detect an image signal from a recording medium on which an image has been recorded (e.g. X-ray film on which an X-ray image has been recorded, or a stimulable phosphor sheet on which a radiation image has been stored with radiation, such as X-rays or cathode rays) In general, in the image read-out apparatus, a light beam is reflected and deflected by a rotating polygon mirror, which has a plurality of reflecting surfaces, such that the light beam scans a recording medium, on which an image has been recorded, in a main scanning direction. At the same time, the recording medium is scanned with the light beam in a sub-scanning direction which is approximately normal to the main scanning direction. When each portion of the recording medium is thus scanned, light representing the image is radiated out of the scanned portion of the recording medium. (For example, in cases where the recording medium is X-ray film, light which has passed through the X-ray film is obtained. In cases where the recording medium is a stimulable phosphor sheet, which is scanned with stimulating rays, it emits light in proportion to the amount of energy stored thereon during its exposure to the radiation.) The light thus radiated out of the recording medium is detected and converted into an image signal by a photoelectric conversion means, which has a light receiving surface, positioned to extend along the main scanning line on the recording medium. (By way of example, the photoelectric conversion means is constituted of a combination of a light guide member with a photomultiplier. The light guide member has a light receiving surface, positioned to extend along the main scanning line on the recording medium, and a ring-shaped light output face. The photomultiplier receives the light emanating from the light output face of the light guide member. Alternatively, as disclosed in, for example, U.S. Pat. No. 4,864,134, the photoelectric conversion means may be constituted of a long photomultiplier, which has a long light receiving surface, positioned to extend along the main scanning line, and which is located close to the recording medium.)
It is necessary that the image signal obtained from the image read-out apparatus accurately represents the image, which was recorded on the recording medium. However, the problem described below often occurs. Specifically, when a recording medium, such as X-ray film or a stimulable phosphor sheet, is uniformly exposed to X-rays or radiation, a uniform image thus being recorded on the recording medium, the recording medium is then scanned with a light beam, and the light, which is radiated out of the recording medium during the scanning and which represents the image, is photoelectrically detected and converted into an image signal by the photoelectric conversion means, the image signal should have uniform value. However, it often occurs that the value of the image signal thus detected is not uniform. The reasons why such a problem occurs will be described hereinbelow.
By way of example, the photoelectric conversion means exhibit certain characteristics of shading. Specifically, even if the amount of light impinging upon the light receiving surface of the photoelectric conversion means is the same, the sensitivity with which the photoelectric conversion means detects the light varies in accordance with the position with respect to the main scanning direction with which the light impinges upon the light receiving surface. During the operation of the image read-out apparatus, little change occurs in the characteristics of the shading unless the photoelectric conversion means is exchanged with a new one.
Also, problems with regard to surface inclination are caused to occur primarily by the deviation in position of the rotation axis from the center axis of the rotating polygon mirror. Specifically, even if the light beam impinges upon the rotating polygon mirror from the same direction, the directions along which the light beam is reflected by different reflecting surfaces of the rotating polygon mirror deviate in the sub-scanning direction. In such a case, the intervals at which a plurality of scanning lines of the light beam are formed on the recording medium, does not become uniform with respect to the sub-scanning direction. As a result, when a visible image is reproduced from the image signal detected from the recording medium, nonuniformity in the image density occurs in the sub-scanning direction for a period corresponding to one turn of the rotating polygon mirror. The extent to which the adverse effects occur from the surface inclination often changes during the operation of the image read-out apparatus.
Nonuniformity in the image density due to the rotating polygon mirror is also caused to occur due to a slight difference in reflectivity or curvature among the reflecting surfaces of the rotating polygon mirror and due to the difference in reflectivity of the various parts of each reflecting surface. Such nonuniformity often changes during the operation of the image read-out apparatus.
In order for the shading, the nonuniformity in the image density, or the like, to be eliminated, a novel method has been proposed in,. for example, U.S. patent application Ser. No. 391,830, now U.S. Pat. No. 4,985,629. With the proposed method, a recording medium is uniformly exposed to X-rays, radiation, or the like. The recording medium, which has been uniformly exposed to the X-rays, the radiation, or the like, is then scanned with a light beam, and the light radiated out of the recording medium during the scanning is photoelectrically detected. In this manner, by way of example, the nonuniformity in sensitivity along the main scanning direction is detected with respect to the respective reflecting surfaces of the rotating polygon mirror. (The nonuniformity in the sensitivity includes not only the nonuniformity in the sensitivity, with which the photoelectric conversion means detects the light radiated out of the recording medium, along the main scanning direction, but also the effects from the nonuniformity in the image density, differences in reflectivity among the reflecting surfaces of the rotating polygon mirror, or the like. All of sources, from which the problems occur in an image signal having uniform value cannot be obtained from a recording medium when the recording medium was exposed uniformly to X-rays, radiation, or the like, will hereinbelow be referred to as nonuniformity in the sensitivity.) The information about the nonuniformity in the sensitivity is stored in a storage means. When an image signal representing an image is detected from a recording medium, on which the image has been recorded, the image signal is corrected in accordance with the information of the nonuniformity in the sensitivity, which is stored in the storage means.
However, often the extent of the surface inclination of the rotating polygon mirror changes during the operation of the image read-out apparatus. As a result, the nonuniformity in the sensitivity changes. Therefore, in order that image signals accurately representing the images be obtained, it is necessary for operations to be carried out, for example, periodically, wherein a recording medium is uniformly exposed to radiation, or the like, and an image signal is detected from the recording medium, which has been uniformly exposed to the radiation, or the like, with the image read-out apparatus. However, considerable time and labor are required to carry out such operations. Also, it is not easy to irradiate radiation, or the like, uniformly to the recording medium. It is possible for the manufacturer of the image read-out apparatus to detect the nonuniformity in the sensitivity by uniformly exposing the recording medium to radiation, or the like, in the course of assembling the image read-out apparatus. However, from the point of view of the required time and labor and the accuracy with which the nonuniformity in the sensitivity is detected, it is difficult to have the user of the image read-out apparatus periodically detect the nonuniformity in the sensitivity.